Introduce subtyping instead of ad-hoc treatment of function arguments.

Instead of using an ad-hoc treatment of function arguments which are callbacks in automatic curried application, use subtyping in uncurried-always mode.
So an uncurried function is cast to a curried one during unification.

In addition to the existing examples, this also supports uses of `|>` and callbacks to `div` such as `onClick`, which now can be uncurried functions.

Author: cristianoc

jscomp/ml/ctype.ml

@@ -2341,6 +2341,9 @@ let rec unify (env:Env.t ref) t1 t2 =
         with Cannot_expand ->
           unify2 env t1 t2
         end
+    | (Tconstr (Pident {name="function$"}, [tFun; _], _), Tarrow _) when !Config.use_automatic_curried_application ->
+        (* subtype: an uncurried function is cast to a curried one *)
+        unify2 env tFun t2
     | _ ->
         unify2 env t1 t2
     end;

jscomp/ml/typecore.ml

@@ -2016,12 +2016,9 @@ and type_expect_ ?in_function ?(recarg=Rejected) env sexp ty_expected =
       end_def ();
       wrap_trace_gadt_instances env (lower_args env []) ty;
       begin_def ();
-      let uncurried, funct =
-        if Ext_list.exists sexp.pexp_attributes (fun ({txt },_) -> txt = "res.uapp")
-        then match is_automatic_curried_application env funct with
-          | Some funct -> false, funct
-          | None -> true, funct
-        else false, funct in
+      let uncurried =
+        Ext_list.exists sexp.pexp_attributes (fun ({txt },_) -> txt = "res.uapp")
+        && not @@ is_automatic_curried_application env funct in
       let (args, ty_res, fully_applied) = type_application uncurried env funct sargs in
       end_def ();
       unify_var env (newvar()) funct.exp_type;
@@ -2116,7 +2113,7 @@ and type_expect_ ?in_function ?(recarg=Rejected) env sexp ty_expected =
       (* Disable Unerasable_optional_argument for uncurried functions *)
       let unerasable_optional_argument = Warnings.number Unerasable_optional_argument in
       Warnings.parse_options false ("-" ^ string_of_int unerasable_optional_argument);
-      let exp = type_construct env loc lid sarg ty_expected sexp.pexp_attributes in
+      let exp = type_construct env loc lid sarg uncurried_typ sexp.pexp_attributes in
       Warnings.restore state;
       exp
   | Pexp_construct(lid, sarg) ->
@@ -2982,25 +2979,10 @@ and type_argument ?recarg env sarg ty_expected' ty_expected =
       texp
 and is_automatic_curried_application env funct =
   (* When a curried function is used with uncurried application, treat it as a curried application *)
-
-  let rec fun_arity texp = match texp.desc with
-    | Tarrow(_, _, t2, _) -> 1 + fun_arity t2
-    | _ -> 0 in
-
-  let rec make_callbacks_uncurried texp = match texp.desc with
-    | Tarrow (lbl, t1, t2, comm) ->
-      let a1 = fun_arity t1 in
-      let t1 = if a1 = 0 then t1 else Ast_uncurried.make_uncurried_type ~env ~arity:a1 t1 in
-      let t2 = make_callbacks_uncurried t2 in
-      {texp with desc = Tarrow (lbl, t1, t2, comm)}
-    | _ -> texp in
-
-  let expanded = expand_head env funct.exp_type in
-  match expanded.desc with
-  | Tarrow _ when !Config.use_automatic_curried_application ->
-    let texp = make_callbacks_uncurried expanded in
-    Some {funct with exp_type = texp}
-  | _ -> None
+  !Config.use_automatic_curried_application &&
+  match (expand_head env funct.exp_type).desc with
+  | Tarrow _ -> true
+  | _ -> false
 and type_application uncurried env funct (sargs : sargs) : targs * Types.type_expr * bool =
   (* funct.exp_type may be generic *)
   let result_type omitted ty_fun =